Electric horns such as those used on automotive vehicles have a plunger/diaphragm assembly which is vibrated by magnetic impulses generated in a coil/pole piece assembly. The displacement of the vibrating diaphragm and the sound output level depends at least in part on the size of an air gap which separates the plunger and the pole piece. Due to manufacturing variations the gap cannot reliably be controlled in a mass production basis unless each horn is calibrated individually.
Automobile horns are mass produced and cost of manufacture is closely controlled and high production fabrication and assembly techniques are employed in their manufacture. The metal housing in which the electromagnetic coil is mounted is formed of drawn sheet metal, and this drawing fabrication step does not result in an accurately dimensioned housing due to variations in the metal ductility, and "spring back". In another embodiment parts are held by a friction fit and are slidably movable. Therefore, due to the difficulty of maintaining accurate manufacturing tolerances, electromagnetic vehicle horns will vary in the level of sound output if adjustment means are not utilized to overcome such tolerance variations.
Adjustment of the air gap between the pole piece and diaphragm plunger by the use of extra components, or machined surfaces, threads, etc. adds significant cost to the manufacture. In U.S. Pat. No. 4,135,473 a method of horn adjustment is disclosed Wherein the axial dimension of the housing is varied by deforming the housing sidewalls by regulating the dimension of a radially extending housing sidewall ridge. The disclosure of this patent permits the horn air gap to be adjusted after the horn components are completely assembled, and operating tests requiring continuous operator evaluation for individual horns are used to determine the correct adjustment. However, the techniques disclosed in this patent are relatively expensive to practice due to the time involved, and the complexity of the apparatus employed.
Another approach to the control of the air gap dimension is presented in the U.S. Pat. No. 4,361,952 to Neese. There it is taught to measure the critical dimensions of parts which affect the air gap and to make the necessary adjustments before assembly. That practice does not take into account the thickness variations of the diaphragm which affect its flexibility and its amplitude of movement. Thus for a fixed nominal gap, a small diaphragm amplitude would result in a larger effective gap at the closest approach of the plunger to the pole piece. Accordingly a functional test which takes into account more factors than the bare dimensions is more appropriate to the optimal adjustment of air gap. Moreover, it is preferable to avoid the measurement of multiple parts if the gap can be obtained in a more direct fashion.
In practice the functional test is practiced by manual adjustment of each horn after assembly. The horn is initially made with a slightly oversize air gap. Then the horn is energized by applying a standard voltage to the coil and a db meter measures and displays the horn output sound level. The operator adjusts the horn gap by forcing the plunger in until the requisite sound level is attained. In a subsequent test the operator increases the applied voltage to a specified voltage limit and listens to the sound quality to determine whether the plunger contacts the pole piece.